docs: def overall_rating(object, category=""):
      """
      Usage:
          {% overall_rating obj [category] as var %}
      """
      try:
          ct = ContentType.objects.get_for_model(object)
          if category:
              rating = OverallRating.objects.get(
                  object_id=object.pk,
                  content_type=ct,
                  category=category_value(object, category)
              ).rating or 0
          else:
              rating = OverallRating.objects.filter(
                  object_id=object.pk,
                  content_type=ct
              ).aggregate(r=models.Avg("rating"))["r"]
              rating = Decimal(str(rating or "0"))
      except OverallRating.DoesNotExist:
          rating = 0
      return rating